In the evolution of physics, its unity is maintained by a network of correspondence principles, through
which simpler theories maintain their vitality by links to more sophisticated but more accurate ones.
The correspondence between the newer theory and its predecessor gives one the ability to recover the
older theory from the newer. In view of Newton's theory of gravitation which is inconsistent with
special relativity, Einstein was published general relativity based on equivalence principle, where the
gravitational field is described by a metric. Furthermore, general relativity will reduce to Newton's
theory of gravity in the limit of weak gravitational fields and low velocities. Einstein [1]}
built the Newtonian limit into general relativity, and also computed the precession of the perihelion
which is one of the post-Newtonian effects. The formalism of Newtonian theory plus post-Newtonian
corrections is called the “post-Newtonian approximation" [2]}.
